I wrote a dialog about this topic and I was hoping you fine people could give me some criticisms. I'm also interested in what you think about the dialog format. I've never tried it before. I think it helps the explanation by presenting clarifying questions, but also seems pretentious. Anyway, all opinions most welcome.

A DIALOGUE!

Q: I’ve been wondering. What exactly is the difference between living things and nonliving things?

R: Usually this question is answered by listing a number of qualities which must be possessed for a thing to be considered alive.

Q: Hm. But I can think of something which satisfies just about all of these, yet we don’t call it alive! The virus!

R: Right, we don’t call them “alive.”

Q: But why not? They reproduce, they evolve and adapt… They sure seem like living things to me. At least, they’re a heck of a lot more alive than a rock is!

R: You’re on to something here! It does seem strange that we would group viruses with rocks instead of, say, bacteria. In fact, what you’re revealing with your question about viruses is that our definitions of “living” and “non-living” matter are strictly human categories into which nature need not fit itself! The categories are far from entirely arbitrary or useless, but they are human inventions nonetheless!

Q: So you’re saying these fuzzy cases like viruses are only tricky because our categories are conceptual constructs overlaid on reality?

R: You catch on quickly! That’s exactly it.

Q: But you did say these categories are useful.

R: I did. They certainly help organize our conceptions of the world. Breaking reality down into simpler pieces or groups (like living/nonliving) helps us to think about them and make predictions. So for instance, we classify both a frog and a tree as living things. Simply by acknowledging that both the frog and the tree belong to the same category, we can assume some things about the tree just by looking at the frog. For instance, we see the frog is made of cells, reproduces, needs energy to function, and so on. A tree has these qualities as well, and we can assume that just from their being in the same category.

Q: I see…

R: And, there certainly is matter which is in many ways very different from that contained in, say, a rock.

Q: How is “living” matter different, then?

R: It’s different in a number of ways. For instance, let’s talk about it from a chemical perspective.

Q: A “chemical perspective?”

R: In terms of the nature of the molecules, the rates and variety of reactions, and so on.

Q: I see… OK, go ahead.

R: So let’s compare two things which anyone would agree are very firmly in their respective corners of the living/nonliving divide. Representing the living is a rabbit. Representing the nonliving is a common rock.

Q: Rabbit – living. Rock – nonliving. Got it.

R: Good. So the chemical behavior of these two things are VERY different. For instance, inside the rabbit, a huge variety of chemical reactions occur, including those involved in digestion, synthesis of new parts, DNA replication, and so on. However, the reactions of the rock are mostly limited to surface atoms dissolving in passing water, or being stripped away by wind or abrasion with other hard objects.

Q: So what it comes down to is the variety of reactions? That’s what really separates living from nonliving?

R: Not just the variety of reactions, but also their rate.

Q: You mean how fast they go.

R: Correct. While the surface reactions involving the atoms of the rock are very very slow, the reactions in the rabbit proceed extremely quickly. This is because the rabbit contains enzymes – large protein or nucleic acid molecules which catalyze (or speed up) reactions by lowering the amount of energy needed for these reactions to proceed. So the rabbit and the rock differ not only in the number of different types of reaction which occur, but also in how fast their respective reactions go.

Q: Hmmm… makes sense.

R: I thought it would!

Q: But I have something that contradicts what you’ve just said.

R: Do you?

Q: Yes. The organisms of the phylum Tardigrada!

R: I had a feeling you were going to bring those up…

Q: Well yes! Because the tardigrades are capable of almost totally suspending their chemical reactions and entering a state of near total inactivity! So when they’re like this, they aren’t that much different from a rock in terms of rate of reaction!

R: Excellent point! So the differences can’t be reduced to differences in chemical behavior.

Q: That’s what it seems to imply to me.

R: Yes, you are right. And there’s a larger point we can make while we’re here. An aside, before we move on.

Q: What’s that?

R: That the reactions of the rock and the reactions of the rabbit are actually both a part of a much larger and more intricate chemical system!

Q: Really? How so?

R: Here’s an example. Say our rock falls into a river. Over time, the rushing water knocks atoms and molecules off of the surface of the rock and they enter the water flow as dissolved ions.

Q: You already mentioned this…

R: I know! Be patient! So the rock’s molecules are now in the water as dissolved ions. Well what if these ions happen to bump into the root of a nice leafy green plant?

Q: The plant will take up the ions and use it!

R: Yes…

Q: … Making these ions a part of a living system…

R: ...Yes…

Q: ...And demonstrating that the distinction between living and nonliving cannot be applied to a given atom in itself, but only in the context of the chemical system the atom is currently participating in! Furthermore, it reveals that everything on Earth from a rock to a plant to a rabbit participates in a single chemical cycle far larger and more complex than any individual component of the system!

R: Well said!

Q: It’s like we’re two aspects of the same being.

R: Indeed!

Q: Quite.

R: So, I trust you are satisfied now?

Q: Absolutely not!

R: No?!

Q: No!! You never resolved the Tardigrada conundrum!

R: You’re right. You know, it seems like you’re looking for some special characteristic of living matter that lets it do all these amazing things like coming back from a decade of inactivity as if nothing had ever happened – some property or quality that the matter in the rabbit has but the matter in the rock doesn’t have.

Q: Yes! That’s exactly what I’m after!

R: Well, I think there is something that will satisfy you. Structure!

Q: Structure?

R: “The beauty of a living thing is not the atoms that go into it, but the way those atoms are put together.” – In other words, their structure!

Q: I don’t follow.

R: So in the rock, the atoms are arranged in a sort of crystal lattice [pic]. It’s more complicated than you might expect, but it’s fairly uniform. Now, in a living thing, the structure is MUCH more complex.

Q: Why? How so?

R: Well first let me give a caveat and reiterate that the matter in a rock is the same sort of stuff as the matter in the rabbit. But the matter in the rabbit is arranged in a much more intricate manner. First of all, it is based on carbon.

Q: Carbon?

R: Yes! Our friend, element #6. Carbon's electron configuration allows it to form long chains and rings to which other atoms or molecules can bind. This allows for a huge variety of different compounds, each of which has its own particular chemical behavior. With the huge diversity of molecules that can be created from a carbon framework, a reaction involving carbon-based matter (and with constant energy input as we get from the sun) can develop to be far, far more complex than a reaction only involving simpler molecules.

Q: Why, exactly?

R: Part of it has to do with size. Let me put the difference in size in perspective - a molecule like water, that's got a molecular weight of about 18 atomic mass units. Not bad, but organic molecules can get huge. Really huge. How huge? Consider hemoglobin, with a molecular weight of 68,000 atomic mass units. Or the colossal protein called connectin, weighing in at 2,993,442 atomic mass units.

Basically, organic molecules can be REALLY REALLY BIG. This huge size allows a tremendous diversity of structures, and this allows a tremendous diversity of chemical reactivities. Such complexity allows structures which remind us greatly of our own mechanical contraptions. And this in turn lets them do complicated things like catalyze chemical reactions.

Q: I think I’m seeing it now… The major difference between living and nonliving matter is the structure of the molecules involved! So the Tardigrade in suspended animation is different from the rock because even when the creature's metabolism slows WAY down, the molecular machinery is still there inside the animal in good form, ready to resume the reaction at full speed when the solvent becomes available!

R: That’s it.

Q: Wow, we’ve covered a lot of ground. Let me try to summarize. Living/nonliving is a human dichotomy which only partly reflects the real divisions among matter. These real divisions include varietions in number of reaction types, the rates of these reactions, and the structure of the involved molecules.

It seemed fine, than the part about velocities is little... incorrect (you can have thousands of fast "inorganic" reaction versus e.g. RUBISCO). Also, just because you're talking about stone, you assume, that in non-living nature the reactions take place only on the surface, but that's not true, in liquid or air (gas), the reactions take place in whole volume of course.